The invention relates to a method for operating a vacuum gripping device comprising a suction gripper unit which is provided with at least one suction gripper and which can selectively be connected, via a control valve device connected to it, to a vacuum source causing a negative pressure in the suction gripper or to a positive pressure source generating an ejector pulse in the suction gripper.
The invention further relates to a vacuum gripping device which can in particular be operated as described above and which comprises a suction gripper unit which is provided with at least one suction gripper and which can selectively be connected, via a control valve device connected to it, to a vacuum source causing a negative pressure in the suction gripper or to a positive pressure source generating an ejector pulse in the suction gripper, wherein an electronic control unit is provided for controlling the control valve device, and wherein at least one pressure measurement device suitable for measuring the actual pressure prevailing in the suction gripper unit and communicating with the electronic control unit is assigned to the suction gripper unit.
From DE10 2007 031 760 A1 a vacuum gripping device constructed and operable as described above is known, which makes it possible to grip via negative pressure and hold objects to be repositioned temporarily and to throw the gripped object off the suction gripper holding it by means of a positive pressure pulse described as ejector pulse after it has been conveyed to a desired location. An electrically selectable control valve device is capable of selectively connecting a suction gripper unit comprising the suction gripper and a working line establishing the connection to the suction gripper either to a vacuum source or to a positive pressure source. For control, the vacuum gripper device is provided with an electronic control unit capable of evaluating an actual pressure measured in the suction gripper unit, for example for continuously monitoring the maintenance of a desired negative pressure level, while the gripped object is being repositioned. The ejector pulse is generated by connecting the suction gripper unit by means of the control valve device to the positive pressure source at a variably adjustable flow rate, so that the time within which the vacuum is built up in the suction gripper and an ejection force of greater or lesser strength is built up can be varied. Unfortunately the required setting parameters are highly dependent on the respective marginal conditions of the system, such as the level of the nominal pressure made available by the positive pressure source or the length of a working line of the suction gripper unit between the control valve device and the suction gripper. This being so, each gripping device has to be individually adapted before the start of operations. Finding the optimum setting is always very time-consuming, in particular because the size and the weight of the objects to be gripped play an important role.
If, for example, the set pulse duration of the ejector pulse is too short and the resulting air flow rate correspondingly low, the vacuum in the suction gripper is not cancelled completely, and the object continues to adhere to the suction gripper. If the pulse duration is too long and the air flow rate to high, on the other hand, the blow-off process can have the result that the ejected object or even other objects in the area is/are blown out of the assigned repository and the objects cannot be deposited in a repeatable manner.
A comparable problem applies to the vacuum gripping device described in DE 10 2008 005 241 A1, in which the ejector pulse is generated in a purely time-controlled manner during an empirically determined, fixedly or adjustably preset time interval.